Doctor blade for coating apparatus



A ril 9, 1963 E. WARNER 3,084,663

DOCTOR BLADE FOR COATING APPARATUS Filed Aug. 29. 1960 Z. O Y

54 INVENTOR.

3,684,663 DOCTOR BLADE FUR COATING APlARATUh Edgar Warner, Middletown, Qhio, assignor to Champion Papers Inc, a corporation of Qhio Filed Aug. 29, 1960, Ser. No. 52,438 8 Claims. Ci. 118-1l18) The present invention relates to the manufacture of coated webs or coated sheet material and has particular application to the manufacture of such webs or sheets wherein the coating composition is an adhesive substance or contains an adhesive which acts to bind the coating substance to the web or sheet or if pigmented, the pigment particles to each other and to a web, a case in point being the manufacture of coated paper for high grade printing purposes. In particular, the invention relates to an improved doctor blade which may be used in such manufacturing processes. The invention may be classified as an improvement over the doctor blade structure shown in Warner Patent 2,598,733 dated June 3, 1952 as well as that disclosed in Warner Patent 2,729,192 dated January 3, 1956.

Since the manufacture of coated paper is an excellent example of the usefulness of the invention and illustrates its advantages, this medium will serve to illustrate the invention, its use and its advantages over known structures, though it will be understood the invention is capable of much broader application and utility.

The manufacture of coated paper by a doctoring process usually comprises the application of an excess of an aqueous dispersion of a mineral pigment and an adhesive to the surface of a moving web and subsequent removal of the surplus over and above the amount (of coating composition) required to form a coating of the desired dry weight on (of coating) the surface of the web. In the processes disclosed in the aforesaid patents, the paper web absorbs aqueous vehicle from the portion of the applied coating layer in contact therewith to form what is therein called a filter cake, which is firm enough to resist the wiping action of the doctor which then rides on the surface of the filter cake and wipes off the overlying fluid coating composition.

When utilizing a doctor coating process in accordance with the aforesaid patents it was found to be difhcult to secure filter cake heavy enough to form coating of the desired dry weight. Attempts have been made to secure the passage of more coating under the doctor by using wire-wound or vertically threaded doctor rods which were intended to pass under the doctor an amount of fluid coating metered by the number, form, and depth of the grooves between the threads or wires. The use of such wire wound or threaded doctor rods with paper coating compositions containing adhesive and mineral pigment was almost always disappointing because of the tendency of the coating to pack firmly into the grooves and reduce the space and the amount of coating passed therethrough.

Additionally, if the spirally wound devices of the prior art are mounted in a close fitting holder, the clogging of the threads tends to build up coating within the holder which can and does actually cause the rod to be displaced from its seat with obviously adverse results. Attempts to cure this difficulty have not proved successful to any great degree because devices capable of cleaning the threads continuously are expensive and in addition to the mentioned difficulties the spiral type of rod has an inherent tendency to creep laterally from the holder.

The present device obviates these dllTlClllilfiS because there is no tendency for the rod to creep laterally and the design of the holder for the rod is such that it acts as a doctoring device to clean the grooved surface thereof thus preventing both build up of coating in the grooves and Efifidfifiii Patented Apr. 9, 1963 lee within the holder. Thus the doctor rod structure of the present invention is self-cleaning, capable of coating uniformly, while at the same time all of the advantages of the prior art devices are retained.

The present structure therefore represents an improvement over the prior art referred to above in that it permits of the application of a greater weight of coating composition to the web being processed than has heretofore been possible with doctor devices, and differs from the fore-running doctoring devices in that this result is obtained by utilizing a relatively small diameter doctor rod provided with a series of alternately arranged annular ribs and grooves on its surface and mounted the rod in a holding structure such that the annular grooves are selfcleaning and are in and of themselves, are doctored of excess coating over and above that which has been deposited upon the web being coated.

The details of the invention will become more apparent from the following description thereof in which reference will be made to the accompanying drawing wherein,

FIG. 1 is a front elevation of the doctor blade comprising the present invention,

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 3,

FIG. 3 is a sectional view similar to P16. 2, but showing a modified form of the invention,

FIG. 4 is a view similar to FIG. 3 but illustrating a further modified form of the invention, and finally FIGS. 5 and 6 are schematic diagrams illustrating the manner in which the invention may be incorporated into several representative coating processes.

Referring now in particular to FIGS. 1 and 2, it will be seen that the doctor blade comprises essentially two parts, the doctor rod 10 and a holder 20 therefor. The doctor rod 10 is formed from a steel rod which has been machined, rolled or otherwise worked by one or the other of several methods to modify its normally smooth cylindrical surface, such that it is comprised of a series of concentric alternately interdigitated circumferential hills 2 and valleys 4 of generally thread-like profile configuration. It is most significant to note that these valleys and grooves are defined as thread-like. They do not comprise a thread in that there is no spiral progression of the valley as is the case of a thread wherein the valley between adjacent ribs would be continuous throughout the length of the article provided with the thread from one end to the other. Rather in the present invention each valley between any two adjacent ribs is an annular recess extending around the circumference of the rod and singularly each circumferential groove is completely independent one from the other. As has been stated the profile configuration of the hills and valleys 2 and 4 may be thread-like in profile appearance. It has been found for example a standard Witworth profile performs a satisfactory function. Of course the thread profile may assume a variety of configurations, for example, the Witworth profile itself may be modified such that the tops of the ribs and the bottoms of the valleys may be slightly rounded rather than as angulated as shown in the drawing. Thus in general, it may be said that the thread-like profile of these alternating ribs and valleys should be such that the included angle between any two adjacent surfaces defining a valley should approximate for most favorable results.

The doctor rod, as has also been stated is preferably manufactured from a steel rod, of about A to /3 inch in diameter, generally of tool steel characteristics. The rod is chromium plated by any suitable process after the described profile has been imparted to its surface. The depth from top of the ribs 2 to the floor of the valleys 4 may be varied for the purpose to be defined hereinafter, though satisfactory results have been obtained when this dimension approximates about '.003-.0O4 inch.

As the profilt depth may be varied so also may the number of ribs per inch length of rod be varied. For example very successful results have been obtained with the specified depth and within the range of from 50 to 150 ribs or threads per inch along the entire length of the doctor rod though this dimension may also vary as will be indicated subsequently.

Again referring to FIGS. 1 and 2, it will be seen that the doctor rod holder may comprise a pair of complimentarily shaped clamping elements 16 and 18. These clamping elements may assume several forms, as illustrated in FIGS. 3 and 4. Each element defines a generally semicircular socket half, such that, when the two elements are joined in complimentary relation they will define a cylindrical socket in which the doctor rod 10 is firmly secured. It is extremely important to note that the terminal edges 22 and 24 of each of the socket defining portions of each holder elements is deformed somewhat in that these edges are bent inwardly to exert a definite pressure against the rod 1%) rather than merely defining a support as does the greater portion of the socket formed by the holder elements 16 and 18. In addition it will also be noted that these edges 22 and 24 are spaced apart by a distance less than the diameter of the rod 10 such that the rod will be physically locked into the holder, yet leaving a substantial portion of the rod exterior exposed for contact with he web or other coating means as the case may be.

It will also be apparent from an inspection of FIGS. 1 and 2, the inwardly directed terminal edges 22 and 24 are provided with serrated inner surfaces comprised of ribs and valleys 32 and 34 and 26 and 28 respectively. The profile configuration of these serrated edges is complementary to that of the doctor rod such that the ribs and 32 respectively ride in each of the valleys 4 provided on the doctor rod 1%. Due to the deformation of the edges 22 and 24 inwardly it will be seen that the serrated edges will always be pressed against doctor rod 10 such that the ribs 28 and 34 thereon will be pressed into close contact within the valleys 4 on the doctor rod it There we various methods to produce the proper configuration of the serrated edges of the holder elements 16 and 18. However, one method of producing the desired close fit between these edges and the doctor rod 16 is derived by manufacturing the holders of stainless steel sheet of suitable forming characteristics which is softer than the rod, assembling the rod 10 in holder 20 and thereafter rotating the rod it Obviously the pressure of the crimped in edges 22 and 24 causes wear on these edges principally at the points of contact between ribs 2 against these edges 22 and 24. As rotation is continued the extreme edges of the holder elements wear until the serrations, ribs 28 and 34-, are gradually worn-in the surface with the result that there is extremely close contact between rod 10 and the edges of holder 20. This extremely close fit is of the utmost importance to the successful operation of the invention.

As has been mentioned the present invention represents an improvement over the prior art, smooth surfaced doctor rods in that it permits of the application of greater coating weights per unit area than any of these devices. This is because the circumferential grooves operate as a series of metering devices allowing coating to pass between the individual chambers formed by these grooves and the web as it contacts the tops of the ribs. Thus the amount of coating applied is a direct function of groove size which is in turn a function of groove depth and the number of thread-like grooves provided per inch of doctor rod length.

It has been found that by application of coating with the present device a known amount of coating weight may be applied in a uniform mannerand at such a solids content that the pattern that might be expected does not form but ratherthe coating completely flows out on the surface of the web within a few seconds after application.

As illustrated in FIG. 1, the rod 10 is driven or rotated sea 4 in holder 26 preferably at an extremely low rate of speed, on the order of from /3 foot per minute surface speed. This enhances the self-doctoring feature of the device on a continuous basis and additionally distributes wear evenly throughout the length of the rod 10. The driving mechanism may be conventional, such as an electric motor coupled to the end of the rod through a speed reduction device. In FIG. 1 a drive mechanism is illustrated schematically as comprising a large sprocket 14 driven from small sprocket 8 via chain or belt 12; the large sprocket being mounted on the end of rod 19; the smaller sprocket being carried on shaft 6 driven by a conventional motor M. If desired the same shaft 6 could be connected to the opposite end of rod 10 such that the rod 10 is driven from both ends simultaneously. This arrangement is desirable to prevent torsional stresses from being developed in the rod 10 where the device is utilized in coating wide webs, such as in the paper making art.

In FiGS. 1 and 2 the doctor device is shown as a socalled rigid doctor rod. As seen the two holder elements l6 and 18 are firmly clamped between a pair of bar members, 30, 30 such that there is little or no fiexure of the rod 14? or holder 29. This arrangement is clearly shown in Patent No. 2,598,733.

On the other hand FIG. 3 illustrates the present invention as utilized as a flexible doctor rod, the holders 16, 18 being riveted or otherwise fixed to a flexible support member 48. This arrangement is clearly shown in Patent No. 2,729,192.

FIG. 4, is a further modification of the structure in that the doctor rod 10 is mounted in holder elements 4-2 and 44 wherein the major portion of the socket defined between them in assembled relation is embodied in holder element 44 and a minor portion in holder 42. Again the terminal ends 45, 46 of these elements 42 and 44 are crimped or deformed inwardly to assure close association of the serrated inner edges 4-7 and 48 with the grooved surface of the rod 10. This arrangement is particularly useful in connection with the flexible mounting arrangement of FIG. 3.

FIGS. 5 and 6 illustrate but several of the many ways of utilizing the invention.

FIG. 5 illustrates a simple tension blade coater wherein the web W passes over a rotating applicator roll 50 which carries coating from pan 51 onto the web, the coating being smoothed and metered to the correct weight as the web thereafter passes under tension over rod 10. In this case either the rigid mounting of FIG. 2, or the flexible mounting means of FIG. 3 may be used.

In FIG. 6, the doctor apparatus is incorporated in a coating apparatus where the web W again contacts an applicator roll 53 rotating in pan 54 Thereafter the web passes through the nip defined between a resiliently covered back-up roll 55 and the doctor blade 10 which preferably will be incorporated in the type of holder as shown in FIG. 4.

From the above description of the invention it will be appreciated that changes and modifications therein are Within the purview of one skilled in the art, all of which are within the spirit and scope of the invention, limited as it is only as defined hereinafter wherein.

What is claimed is:

1. A doctor device for the application of coating compositions to moving webs and sheets comprising a doctor rod, a flexible holder for said rod, support means ai'fixed to said holder, said doctor rod having a surface comprised of a series of closely spaced alternately arranged circumferential ribs and grooves, said holder defining a partial elongated, channel-like socket receiving said rod throughout its length and having spaced apart terminal edges only conforming to the surface profile of said rod and in close engagement therewith and spaced apart a distance less than the diameter of the rod when said rod is emplaced within said socket.

2. A doctor device as defined in claim 1 including means for rotating said rod in said socket.

3. A doctor device as defined in claim 1, wherein said support means is rigid.

4. A doctor device as support means is flexible.

5. A doctor device as defined in claim 1 wherein said holder is comprised of a pair of complementary elements joined together to define a partial socket.

6. A doctor device for the application of coating compositions to moving webs and sheets comprising a doctor rod, a holder for said rod, said rod having a thread-like surface profile defined by alternatingly and successively arranged, circumferential grooves and ribs, the grooves and ribs being of generally V-shaped cross-sectional configuration such that the included angle defined between any two adjacent surfaces defined a groove is approximately 90, said holder comprising an elongated socket receiving said rod throughout its length and embracing a major portion thereof and having a pair of parallel terminal edges spaced apart a distance less than the diamdefined in claim 1 wherein said eter of said rod to expose a minor portion of the surface thereof to define a working surface, the terminal edges only of said socket being provided with a serrated confiiguration corresponding to the profile configuration of said rod and being in close engagement with all points on the surface of said rod.

7. A doctor device for the application of coating compositions to moving sheets and webs as defined in claim 6 wherein the depth of the grooves defined between any two adjacent ribs is from .003 to .004 inch.

8. A doctor device as defined in claim 6 wherein the surface of said doctor rod is provided from 50 to 150 circumferential ribs per each inch of length.

References Cited in the file of this patent UNITED STATES PATENTS 1,980,961 Sommer Nov. 13, 1934 2,598,733 Warner June 3, 1952 2,729,192 Warner Ian. 3, 1956 

1. A DOCTOR DEVICE FOR THE APPLICATION OF COATING COMPOSITIONS TO MOVING WEBS AND SHEETS COMPRISING A DOCTOR ROD, A FLEXIBLE HOLDER FOR SAID ROD, SUPPORT MEANS AFFIXED TO SAID HOLDER, SAID DOCTOR ROD HAVING A SURFACE COMPRISED OF A SERIES OF CLOSELY SPACED ALTERNATELY ARRANGED CIRCUMFERENTIAL RIBS AND GROOVES, SAID HOLDER DEFINING A PARTIAL ELONGATED, CHANNEL-LIKE SOCKET RECEIVING SAID ROD THROUGHOUT ITS LENGTH AND HAVING SPACED APART TERMINAL EDGES ONLY CONFORMING TO THE SURFACE PROFILE OF SAID ROD AND IN CLOSE ENGAGEMENT THEREWITH AND SPACED APART A DISTANCE LESS THAN THE DIAMETER OF THE ROD WHEN SAID ROD IS EMPLACED WITHIN SAID SOCKET. 